1. Field of the Invention
This invention relates to door locks, and more particularly, to a push-type lock for fire-blocking doors that are used in the event of a fire to prevent the fire from spreading to the emergency staircase.
2. Description of Related Art
A prior art push-type lock for fire-blocking doors is shown in FIGS. 1 and 2. This push-type lock is composed of a casing 10', a frame 11', a mount 20', a push plate 30', an actuating piece 15', and a pivotal latching member 16'. The pivotal latching member 16' has a rear end formed with a protruding piece 19' having a tip abutting on the free end 151' of the actuating piece 15'. A movable arm 24' and a stop pin 17' are coupled to the underneath of the actuating piece 15'. The free end of the stop pin 17' can be propelled by a spring 18' into a slot 161' formed on the back of the pivotal latching member 16' and in which case the pivotal latching member 16' is stopped by the stop pin 17 and thus cannot be withdrawn to the inside of the casing 10'. When a person wants to open a door installed with the push-type lock, he/she can press the push plate 30' by hand. This causes the actuating piece 15' to be pivoted in a direction as indicated by the arrow in FIG. 2, thereby impelling the movable arm 24' and the stop pin 17', and thereby withdrawing the free end of the stop pin 17' away from the slot 161' on the back of the pivotal latching member 16'. As a result of this, the free end 151' of the actuating piece 15' is impelled upwards, thereby urging against the protruding piece 19' of the pivotal latching member 16' and causing the same to pivot in the clockwise direction (with reference to the side view of FIG. 2) . This action causes the pivotal latching member 16' to withdraw from the outside of the casing 10' to the inside thereof, thus allowing the door currently being latched by the pivotal latching member 16' to be unlatched. In normal condition, the door is latched by the push-type lock. However, it is a drawback of this prior art push-type lock that when a fire breaks out, the constituent parts of the push-type lock could be deformed due to the heat from the fire. For instances, the stop pin 17' could be heated to bend and thus has its free end withdraw from the slot 161', or the actuating piece 15' could be deformed that causes the free end 151' thereof to be disengaged from the pivotal latching member 16'. All of these could cause the pivotal latching member 16' to be unable to latch the door securely, or result in the withdrawal of the pivotal latching member 16' to the inside of the casing 10'. The door thus could be pushed open by the explosive, heated air from the fire, thus adversely causing the fire to spread to the next, room.
There exists, therefore, a need for an improved push-type lock for fire-blocking doors that will not be deformed due to heat and notwithstanding latch the door securely when a fire breaks out.